Open-angle glaucoma arises from serious damage to the optic nerve and is strongly correlated with high intraocular pressure (IOP). High levels of IOP can lead to optic nerve damage, glaucoma, and eventually, blindness. Glaucoma is a very serious public health problem. This is exacerbated by the fact that in the vast majority of cases there are no early symptoms; by the time that loss of peripheral vision starts to become noticeable, irreversible damage to the optic nerve has already occurred. More than 2.2 million Americans age 40 plus are potentially at risk. (Friedman 2002). Indeed, an estimated three million people in the United States have the disease and as many as 120,000 are blind as a result. In addition, glaucoma is the number one cause of vision loss in African Americans, and Hispanics are also at higher risk of glaucoma.
Treatments to slow the progression of the disease are available. High IOP is the only significant modifiable risk factor. In fact, it has recently been rigorously established that lowering of IOP in individuals with glaucoma correlates with a delay in the onset of optic nerve damage and peripheral visual field loss. The same has also been recently established for a subset of patients with ocular hypertension, though this subset is not well characterized.
There are currently several instruments available for measuring IOP. The most widely used instrument is the Goldmann Applanation Tonometer (GAT), with the resulting readings of IOP being referred to as IOPG. Other instruments in use include the TonoPen and pneumo-tonometer (puff tonometer). It is widely known and generally accepted in the ophthalmology profession that these instruments have inherent systematic errors. These errors are generally attributed primarily to the effect of corneal stiffness which is unaccounted for in applanation tonometry.
Since IOP is used as the main screening mechanism for early detection of those at risk, it is evident that accurate measurement of IOP is of primary importance: false low IOP readings may delay or prevent necessary treatment, with devastating effects up to and including blindness; false high IOP readings may have undesirable consequences in the form of unnecessary and costly lifelong therapy. In light of this, it is clear that increased accuracy and reliability of IOP measurement has great public health value; indeed, the medical management of individuals at risk for glaucoma could be significantly enhanced.
Clinical studies have repeatedly shown that IOPG measurements with the GAT are influenced by the Central Corneal Thickness (CCT). Several authors have demonstrated the correlation between IOPG and CCT using data from simultaneous cannulation and applanation tonometry. The early study by Ehlers provided a table to recalculate IOPG taking account of the CCT of a patient. Ehlers, N., Bramsen, T., and Sperling, S., Applanation Tonometry and Central Corneal Thickness, Acta Ophthalmologica, 53(1), pp. 34-43 (1975). Many other references have also discussed the effect of CCT on IOP measurement.
On the other hand, data from clinical studies also shows considerable scatter in the correlation between IOPG and CCT. One implication of the observed scatter is that the true IOP, which is acknowledged to depend on corneal stiffness through CCT, also depends on the material or mechanical properties of the cornea. A correction which takes into account only the CCT, according to currently recommended practice, may not be sufficient.